The present invention relates to a process and apparatus for thermal cracking of liquid or gaseous hydrocarbons, ranging from ethane to gas oil, crude oil, residual oils or tar sands and more particularly to a non-catalytic cracking process employing a pressurized riser-type thermal cracker heated by hot agglomerated ash particles circulated from a separate coal burning power producing combustion unit.
The core of the petroleum chemical industry is represented by the production of the lower olefins. In this country ethane cracking has heretofore been considered the most practical way to produce ethylene, and the ethane can readily be extracted from "wet" natural gas available here.
In European countries which do not have a ready supply of such "wet" natural gas, the ethane is more costly and chemicals such as ethylene, propylene and valuable coproducts have been produced by naptha cracking. This is a less economical way of producing ethylene than by ethane cracking. However, the values of the coproducts are becoming better appreciated in this country, and the desirability of naphtha cracking increases as the long term availability of ethane for cracking to ethylene becomes more questionable.
Because naphtha is not widely available in this country for steam cracking, heavier feedstocks, such as gas oil provide an attractive source, and there has long been a need for an efficient economical process for producing lower olefins from the heavier feedstocks. Various systems have been proposed, but they have had serious shortcomings and have not provided the most economical way to affect thermal cracking of the heavy feedstock.
One problem when using feedstocks such as gas oil, crude oil or residual oils, is that coke or char forms during pyrolysis and tends to foul up the equipment. In one proposed fluidized bed cracking process, such fouling is avoided because the coke particles formed in the bed are spatially separated from the cracking operation and burned in the lower part of the reactor. In a proposed "fluidized-flow" process which uses inert ceramic heat-carrier particles as the heat transfer media, the ceramic particles become coated with coke or char in the fluidized bed of the pyrolysis unit. These ceramic particles are continually removed from the bottom of the unit and the coke thereof is burned with additional fuel in a separate burner unit to reheat the particles while removing the coke or char therefrom.
In conventional plants for naphtha cracking, the pyrolysis unit is heated exterally using oil-fired or gas-fired furnaces for supplying heat. The various proposed alternative methods of supplying heat such as heated ceramic particles, sand or refractory checker work, high-temperature steam and flame cracking techniques do not appear to be commercially attractive at the present time.
During the last decade there have been unprecedented increases in the capital costs for chemical plants and further costs involved in reducing air pollution and meeting other environmental requirements. There has been an increasing need for the economies possible in chemical plants of large size. When such factors are taken into consideration, few processes are sufficiently efficient and economical to meet the requirements for a commercially attractive chemical plant.
The process of the present invention can meet those requirements because of its high efficiency and also because it uses a readily available feedstock, such as gas oil, residual oils, tar sands or diatomaceous earth containing oil. In the past, residual oils such as reduced crude or vacuum residuum could not be processed by a petroleum refiner in a fluid bed catalytic cracker because of the high Conradson carbon of the oil and mainly because of the unusually high metals content of these feedstocks which tend to poison the catalyst. The process of this invention could replace the cat cracker and thus provide the refiner with an ethylene unit and a gasoline and fuel oils producer utilizing vacuum residuum as a feedstock. Because ash is produced in the process, the continuous purge of ash can effectively handle the metals problem whereas an expensive catalyst becomes poisoned by the metals. The metals can be recovered from the ash purged if desired.
The high Conradson carbon of these residual oils tends to upset the heat balance of the fluid cat cracker and some means must be provided to remove the added heat generated from burning of the high coke laydown on the catalyst. By the process of this invention, it is only necessary to reduce the coal feed to the burner to keep the unit in heat balance.
Although ethylene yield may be limited in such an operation because of the refractory type feedstock handled, the process is advantageous and will provide a means for producing other upgraded products such as gasoline and mid distillates which heretofore could not be economically produced from vacuum residuum except by an expensive coking process.
Heretofore, various methods have been proposed for recovering hydrocarbons from tar sands but the recovered hydrocarbons have been too expensive to compete with petroleum crudes recovered by more conventional methods, particularly because of the difficulty in processing large volumes of sand. Also the oil obtained from tar sands has been considered less valuable because it is heavier and more viscous than conventional petroleum crude. The present invention permits economical production of ethylene and valuable coproducts from tar sand and makes possible efficient processing, separation and disposal of the sand.